1. Field of the Invention
The present invention generally relates to semiconductor integrated circuits, and particularly relates to a semiconductor integrated circuit which has cross-talk noise thereof reduced by shielding wire lines.
2. Description of the Related Art
In electric circuits, inductance coupling and capacitance coupling between signal lines cause cross-talk noise. The greater the inductance coupling and capacitance coupling between lines, the greater the cross-talk noise is. In semiconductor devices such as large-scale integrated circuits, inductance coupling and capacitance coupling between lines increase as the circuit density increases, thereby causing noise to appear more conspicuously. In order to reduce cross-talk noise caused by the inductance and capacitance coupling, shielding wire lines are used.
Use of shielding wire lines gives rise to problems in that they have relatively large wiring resistance per unit length.
First, large wiring resistance results in electric charging and discharging in the shielding wire lines being slowed, thereby reducing a shielding effect.
Second, when a shielding wire line is connected to the ground voltage or to the power supply voltage at several points along the line, an excessively large current runs through the shielding wire line, causing the problem of E-MIG (electro migration). The E-MIG occurs when the gradient of a power supply voltage in the LSI is ignored. For example, the power supply voltage differs between a point close to the power supply pin of the LSI and the center point of the LSI. If a shielding wire line is laid out between the points having a voltage difference, and is clipped to the power supply voltage at two end points thereof, an excessively large current ends up running through the shielding wire line. In order to circumvent this, analysis of power supply networks may be conducted to detect points where requirements of current density are not met, followed by making corrections one by one. Since there is no guarantee that this will converge, a time spent on the process of analyzing and making corrections would be regarded as a problem, especially when the speed of product development is given high priority.
Third, conventional shielding is made by providing a shield along all the extent of a shielded line. As a result, shielding wire lines consumes a wiring area to a large extent, giving rise to a problem in that an efficient layout becomes difficult to achieve. To obviate this, such measures as increasing the number of wiring layers or enlarging chip size may be conceivable. It is desirable, however, to provide efficient shielding wire lines under the given limitations of the number of wiring layers and chip size.
Accordingly, there is a need for a method of laying out effective and efficient shielding wire lines and a semiconductor integrated circuit that has effective and efficient shielding wire lines.
It is a general object of the present invention to provide a semiconductor integrated circuit that substantially obviates one or more of the problems caused by the limitations and disadvantages of the related art.
Features and advantages of the present invention will be set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by a semiconductor integrated circuit particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a semiconductor integrated circuit, including a shielded wire line and a shielding wire line provided for the shielded wire line and divided into a plurality of segments in a longitudinal direction of the shielded wire line.
In the invention described above, the shielding wire line is divided into a plurality of segments, which makes smaller an effect of wiring resistance, thereby speeding up the electric charging and discharging of the shielding wire line. This improves the shielding effect. Further, division into the plurality of segments eliminates a need to connect a shielding wire line to a power supply potential or a ground potential at two positions far apart in the semiconductor integrated circuit. This obviates the problem of E-MIG.
According to another aspect of the present invention, the semiconductor integrated circuit as described above is such that at least one of the plurality of segments is connected to a first power supply potential, and remaining ones of the plurality of segments are connected to a second power supply potential different from the first power supply potential.
In the invention described above, the shielding wire line provided for the shielded wire line is connected to both the power supply potential and the ground potential. This configuration allows clippings to be made at a relatively large number of points, compared with a configuration in which shielding wire lines provided for a shielded wire line are connected to only either the ground potential line or the power supply potential line as in the case of conventional shielding wire lines. Provision of clippings at a larger number of points makes an effect of wire resistance lower, thereby increasing a shielding effect.
According to another aspect of the present invention, the semiconductor integrated circuit as described above is such that at least one of the plurality of segments is connected to a power supply potential only at a single connection point.
In the invention described above, the shielding wire line provided for the shielded wire line is divided into the plurality of shielding segments, each of which is fixed to a certain potential at a single clipping position. Since each shielding segment is provided with a potential through a single clipping position, the problem of E-MIG is eliminated.
Further, the present invention provides a semiconductor integrated circuit, including a shielded wire line and a shielding wire line provided for the shielded wire line and having a width broader than that of the shielded wire line.
Moreover, the present invention provides semiconductor integrated circuit, includes a shielded wire line and a plurality of shielding wire lines provided for the shielded wire line on one side of the shielded wire line.
Use of a broader shielding wire line or multiple shielding wire lines improves an shielding effect.
According to another aspect of the present invention, a semiconductor integrated circuit includes a shielded wire line and a shielding wire line provided along only a portion of an entire extent of the shielded wire line.
In the invention described above, a shielding wire line is provided only for a portion of an entire extent of the shielded wire line, thereby achieving shield wiring that makes an efficient use of wiring areas.
Further, the semiconductor integrated circuit as described above further includes a driver that transmits a signal to the shielded wire line, wherein the portion of the entire extent of the shielded wire line along which the shielding wire line is provided is a portion on a side of the driver.
In the invention described above, the shielding wire line is provided only for a portion of the entire extent of the shielded wire line on a side of the driver. A signal propagating through the shielded wire line has such a rising edge that the closer it is to the driver, the steeper the slope is. Accordingly, the effect of cross-talk noise that the shielded wire line on the noise source side has on the shielded wire line on the noise recipient side is the strongest near the driver, and becomes increasingly weak as it increases the distance from the driver and comes closer to the receiver. If a portion of the extent of the shielded wire line is shielded on the side of the driver, therefore, the effect of cross-talk noise can be sufficiently reduced.